Contact sensors and methods for making same

ABSTRACT

Disclosed herein are novel contact sensors. The contact sensors disclosed herein include a conductive composite material formed of a polymer and a conductive filler. In one particular aspect, the composite materials can include less than about 10 wt % conductive filler. Thus, the composite material of the contact sensors can have physical characteristics essentially identical to the polymer, while being electrically conductive with the electrical resistance proportional to the load on the sensor. The sensors can provide real time dynamic contact information for joint members under conditions expected during use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/157,963, filed on Mar. 6, 2009, which application is hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to contact sensors, and more particularly tocontact sensors for accurately measuring surface contact data at ajunction between two members.

2. Description of the Related Art

Contact sensors have been used to gather information concerning contactor near-contact between two surfaces in medical applications, such asdentistry, podiatry, and in the development of prostheses, as well as inindustrial applications, for example to determine load and uniformity ofpressure between mating surfaces, and in the development of bearings andgaskets. In general, these sensors include pressure-sensitive filmsdesigned to be placed between mating surfaces. These film sensors, whilegenerally suitable for examining static contact characteristics betweentwo generally flat surfaces, have presented many difficulties in othersituations. For example, when examining contact data between morecomplex surfaces, surfaces including complex curvatures, for example, itcan be difficult to conform the films to fit the surfaces withoutdegrading the sensor's performance.

More serious problems exist with these materials as well. For example,film-based contact sensor devices and methods introduce a foreignmaterial having some thickness between the mating surfaces, which canchange the contact characteristic of the junction and overestimate thecontact areas between the two surfaces. Moreover, the ability to examinereal time, dynamic contact characteristics is practically non-existentwith these types of sensors.

A better understanding of the contact conditions at joints and junctionscould lead to reduced wear in materials, better fit between matingsurfaces, and longer life expectancy for machined parts. For example,one of the leading causes of failure in total joint replacementprostheses is due to loosening of the implant induced by wear debrisparticles worn from the polymeric bearing component. A betterunderstanding of the contact conditions between the joint componentswould lead to reduced implant wear and longer implant life.

What are needed in the art are contact sensors that can provide moreaccurate and/or dynamic contact information concerning a junction formedbetween two surfaces of any surface shape.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a contact sensor.The sensor includes an electrically conductive composite materialcomprising a polymer and a conductive filler. Generally, the compositematerial can include any polymer. In certain aspects, the polymer can bean engineering polymer or a high performance polymer. In one preferredaspect, the composite material can include ultra-high molecular weightpolyethylene (UHMWPE). In one aspect, the composite material of thesensors can include between about 0.1% and 20% by weight of a conductivefiller. The conductive filler can be any suitable material. For example,in one aspect, the conductive filler can include carbon black.

The contact sensors of the invention can define a contact surface.Optionally, a surface of the contact sensors of the invention can beformed to be substantially inflexible so as to replicate a surface thatcan be placed in proximity to a surface of a second member, therebyforming a junction. In particular, the contact surface of the sensors ofthe invention can replicate the shape and optionally also the materialcharacteristics of a junction-forming member found in an industrial,medical, or any other useful setting. For example, in one particularaspect, the essentially inflexible contact surface of the sensor caninclude curvature such as that described by the contact surface of apolymeric bearing portion of an implantable artificial replacement jointsuch as the polymeric bearing portion of a hip, knee, or shoulderreplacement joint. Alternatively, the contact sensors can bethermoformed into a desired substantially inflexible three-dimensionalshape. For example, the contact sensors can be thermoformed for use as aprosthetic device.

In one aspect, the sensor can be formed entirely of the compositematerial. In another aspect, the contact sensors of the invention caninclude one or more discrete regions of the electrically conductivecomposite material and a non-conductive material. For example, thesensors can include multiple discrete regions of the electricallyconductive composite material that can be separated by an interveningnonconductive material, e.g., an intervening polymeric material. In oneparticular aspect, the intervening polymeric material separatingdiscrete regions of the composite material can include the same polymeras the polymer of the electrically conductive composite material.

In another aspect, the sensor can comprise one or more sensing points.The sensing points can be configured to measure current flowtherethrough the sensing point during application of a load. In oneaspect, the current flow measured at each sensing point can betransmitted to a data acquisition terminal In an additional aspect, thedata acquisition terminal can transmit a digital output signalindicative of the current flow measurements to a computer having aprocessor. In a further aspect, the processor can be configured tocalculate the load experienced at each respective sensing point usingthe digital output signal. In this aspect, the computer can beconfigured to graphically display the loads experienced at the sensingpoints as a pressure distribution graph. It is contemplated that thepressure distribution graph can be a three-dimensional plot or atwo-dimensional intensity plot wherein various colors correspond toparticular load values. It is further contemplated that the computer canbe configured to display the pressure distribution graph substantiallyin real-time. In still a further aspect, the computer can be configuredto store the load calculations for the plurality of sensing points forfuture analysis and graphical display.

In one aspect, the electrically conductive composite material can belocated at the contact surface of the sensor for obtaining surfacecontact data. If desired, the sensor can include composite material thatcan be confined within the sensor, at a depth below the contact surface,in order to obtain internal stress data.

The electrically conductive composite material described herein can, inone particular aspect, be formed by mixing a polymer in particulate formwith a conductive filler in particulate form. According to this aspect,in order to completely coat the polymer granules with the granules ofthe conductive filler, the granule size of the polymer can be at leastabout two orders of magnitude larger than the granule size of theconductive filler. For example, the average granule size of the polymercan, in one aspect, be between about 50 μm and about 500 μm. The averagegranule size of the conductive filler can be, for example, between about10 nm and about 500 nm.

Following a mixing step, the composite conductive material can be formedinto the sensor shape either with or without areas of non-conductivematerial in the sensor, as desired, by, for example, compressionmolding, RAM extrusion, or injection molding. If desired, a curvaturecan be formed into the contact surface of the sensor in the molding stepor optionally in a secondary forming step such as a machining or cuttingstep.

During use, the sensors of the invention can be located in associationwith a member so as to form a contact junction between a surface of themember and the contact surface of the sensor. The sensor can then beplaced in electrical communication with a data acquisition terminal, forexample via a fixed or unfixed hard-wired or a wireless communicationcircuit, and data can be gathered concerning contact between the sensorand the member. In one particular aspect, dynamic contact data can begathered. For example, any or all of contact stress data, internalstress data, load, impact data, lubrication regime data, and/orinformation concerning wear, such as wear mode information can begathered.

In another aspect, the disclosed sensors can be integrated with the partthat they have been designed to replicate and actually used in the jointin the desired working setting. For example, the contact sensor cangather data while functioning as a bearing of a joint or junction inreal time in an industrial, medical, or other working setting.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the instantinvention and together with the description, serve to explain, withoutlimitation, the principles of the invention. Like reference charactersused therein indicate like parts throughout the several drawings.

FIG. 1 illustrates one aspect of the sensor disclosed herein forobtaining surface contact data of a junction;

FIG. 2 illustrates another aspect of the sensor disclosed herein forobtaining surface contact data of a junction;

FIG. 3 illustrates another aspect of the sensor disclosed herein forobtaining sub-surface contact data of a junction;

FIG. 4 illustrates another aspect of the sensor disclosed herein forobtaining pressure data of a junction, showing two stacked sensorsheets, each sheet having a plurality of spaced conductive stripes, thestacked sensor sheets being oriented substantially perpendicular to eachother such that an array of sensing points is formed by the overlappingportions of the conductive stripes of the stacked sensor sheets;

FIG. 5 illustrates a simplified, non-limiting block diagram showingselect components of an exemplary operating environment for performingthe disclosed methods;

FIG. 6 graphically illustrates the stress v. strain curve for exemplarycomposite conductive materials as described herein;

FIG. 7 graphically illustrates the log of resistance vs. log of the loadfor three different composite conductive materials as described herein;

FIG. 8 illustrates the log of normalized resistance vs. log of the loadfor three different composite conductive materials as described herein;

FIG. 9 illustrates the voltage values corresponding to load, position,and resistance of an exemplary composite material;

FIGS. 10A-10D illustrate the kinematics and contact area for exemplaryartificial knee implant sensors as described herein with differentsurface geometries;

FIGS. 11A and 11B graphically illustrate the log of normalizedresistance vs. log of the compressive force for two different compositeconductive materials as described herein;

FIG. 12 is a photograph of one aspect of an exemplary mold and pressused to form sensor sheets as disclosed herein;

FIG. 13 is a photograph of a sensor sheet according to one aspectdisclosed herein, illustrating a plurality of dots comprising aconductive filler;

FIG. 14 is a schematic of a contact sensor in operative communicationwith a data acquisition terminal, and showing a battery operativelycoupled to the data acquisition terminal and a computer coupled to thedata acquisition terminal via a Wi-Fi transmitter;

FIG. 15 is a schematic of an exemplary interface circuitry for the dataacquisition terminal; and

FIG. 16 schematic of an exemplary measurement circuitry for the dataacquisition terminal

DEFINITIONS OF TERMS

For purposes of the present disclosure, the following terms are hereindefined as follows:

The term “primary particle” is intended to refer to the smallestparticle, generally spheroid, of a material such as carbon black.

The term “aggregate” is intended to refer to the smallest unit of amaterial, and in particular, of carbon black, found in a dispersion.Aggregates of carbon black are generally considered indivisible and aremade up of multiple primary particles held together by strong attractiveor physical forces.

The term “granule” is also intended to refer to the smallest unit of amaterial found in a dispersion. However, while a granule can also be anaggregate, such as when considering carbon black, this is not arequirement of the term. For example, a single granule of a polymer,such as UHMWPE or conventional grade polyethylene, for example can be asingle unit.

The term “agglomeration” is intended to refer to a configuration of amaterial including multiple aggregates or granules loosely heldtogether, as with Van der Waals forces. Agglomerations of material in adispersion can often be broken down into smaller aggregates or granulesupon application of sufficient energy so as to overcome the attractiveforces.

The term “conventional polymer” is intended to refer to polymers thathave a thermal resistance below about 100° C. and relatively lowphysical properties. Examples include high-density polyethylene (PE),polystyrene (PS), polyvinyl chloride (PVC), and polypropylene (PP).

The term “engineering polymer” is intended to refer to polymers thathave a thermal resistance between about 100° C. and about 150° C. andexhibit higher physical properties, such as strength and wearresistance, as compared to conventional polymers. Examples includepolycarbonates (PC), polyamides (PA), polyethylene terephthalate (PET),and ultrahigh molecular weight polyethylene (UHMWPE).

The term “high performance polymer” is intended to refer to polymersthat have a thermal resistance greater than about 150° C. and relativelyhigh physical properties. Examples include polyetherether ketone (PEEK),polyether sulfone (PES), polyimides (PI), and liquid crystal polymers(LC).

Contact stress, synonymous with contact pressure, is herein defined assurface stress resulting from the mechanical interaction of two members.It is equivalent to the applied load (total force applied) divided bythe area of contact.

Internal stress refers to the forces acting on an infinitely small unitarea at any point within a material. Internal stress varies throughout amaterial and is dependent upon the geometry of the member as well asloading conditions and material properties.

Impact force is herein defined to refer to the time-dependent force oneobject exerts onto another object during a dynamic collision.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, and claims, and their previousand following description. Before the present system, devices, and/ormethods are disclosed and described, it is to be understood that thisinvention is not limited to the specific systems, devices, and/ormethods disclosed unless otherwise specified, as such can, of course,vary. It is also to be understood that the terminology used herein isfor the purpose of describing particular aspects only and is notintended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its best, currently known aspect. Thoseskilled in the relevant art will recognize that many changes can be madeto the aspects described, while still obtaining the beneficial resultsof the present invention. It will also be apparent that some of thedesired benefits of the present invention can be obtained by selectingsome of the features of the present invention without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations to the present invention are possibleand can even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “sensor” includes aspects having two or moresensors unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes examples where said event or circumstanceoccurs and examples where it does not.

Presented herein are contact sensors, methods of forming contactsensors, and methods of advantageously utilizing the sensors. Ingeneral, contact sensors can be utilized to gather dynamic and/or staticcontact data at the junction of two opposing members such as a junctionfound in a joint, a bearing, a coupling, a connection, or any otherjunction involving the mechanical interaction of two opposing members,and including junctions with either high or low tolerance values as wellas junctions including intervening materials between the members, suchas lubricated junctions, for example. Dynamic and/or static data thatcan be gathered utilizing the disclosed sensors can include, forexample, load data, lubrication regimes, wear modes, contact stressdata, internal stress data, and/or impact data for a member forming thejunction. The contact sensors disclosed herein can provide extremelyaccurate data for the junction being examined, particularly in thoseaspects wherein at least one of the members forming the junction in theworking setting (as opposed, for example, to a testing setting) isformed of a polymeric material.

Beneficially, the sensors described herein can be configured toreplicate either one of the mating surfaces forming the junction.Optionally, the sensors described herein can be essentially inflexiblewhen positioned proximate the junction. As such, in a laboratory-typetesting application, the sensor can simulate one member forming thejunction, and contact data can be gathered for the junction underconditions closer to those expected during actual use, i.e., withoutaltering the expected contact dynamics experienced at the junctionduring actual use. For example, the disclosed sensors can providecontact data for the junction without the necessity of includingextraneous testing material, such as dyes, thin films, or the like,within the junction itself.

In one aspect, the sensor can be formed of a material that essentiallyduplicates the physical characteristics of the junction member that thesensor is replicating. Accordingly, in this aspect, the sensor canexhibit wear characteristics essentially equivalent to those of themember when utilized in the field, thereby improving the accuracy of thetesting data. According to one particular aspect of the invention,rather than being limited to merely simulating a junction-formingmember, such as in a pure testing situation, the sensor can beincorporated into the member itself that is destined for use in theworking application, i.e., in the field, and can provide contact datafor the junction during actual use of the part. It is contemplated thatthe sensors described herein can be used in a variety of workingsettings, including, for example and without limitation, in industrialworking settings, medical working settings, and the like.

In an additional aspect, the contact sensors disclosed herein can beformed to be substantially inflexible. In this aspect, it iscontemplated that the contact sensors can be thermoformed as desiredinto a three-dimensional shape. In one aspect, it is contemplated thatthe desired shape of the contact sensors can be a substantiallysheet-like member. Optionally, the desired shape of the contact sensorscan substantially replicate the three-dimensional shape of a selectedstructure of a subject's body, including, for example and withoutlimitation, a bone, limb, or other body member. Accordingly, it iscontemplated that the contact sensors can be thermoformed to function,for example and without limitation, as prosthetic devices for use as areplacement for, or in conjunction with, the selected structure of thesubject's body. It is further contemplated that the desired shape of thecontact sensors can substantially replicate the three-dimensional shapeof a selected structure outside the body of a subject that is configuredto bear loads, including, for example and without limitation, textiledevices, vehicle parts and components, anthropomorphic test devices suchas crash test dummies, building components, and the like.

In various aspects, the contact sensors disclosed herein can comprise anelectrically conductive composite material that in turn comprises atleast one non-conductive polymer material combined with an electricallyconductive filler. In another aspect, the composite material disclosedherein can comprise an electrically conductive filler that can providepressure sensitive electrical conductivity to the composite material,but can do so while maintaining the physical characteristics, e.g., wearresistance, hardness, etc., of the non-conductive polymeric material ofthe composite. Thus, in this aspect, the sensors disclosed herein can bedeveloped to include a particular polymer or combination of polymers soas to essentially replicate the physical characteristics of the similarbut non-conductive polymeric member forming the junction orthree-dimensional structure to be examined

This combination of beneficial characteristics in the compositematerials has been attained through recognition and/or development ofprocesses for forming the composite materials in which only a smallamount of the electrically conductive filler need be combined with thepolymeric material. As such, the physical characteristics of thecomposite material can more closely resemble those of the startingpolymeric material, and the sensor can closely replicate the physicalcharacteristics of a non-conductive polymeric member forming a junction.

This feature can be particularly beneficial when considering theexamination of junctions including at least one member formed ofengineering and/or high performance polymers. When considering suchmaterials, the addition of even a relatively small amount of additive orfiller can drastically alter the physical characteristics that providethe desired performance of the materials. In the past, when attemptswere made to form electrically conductive composites of many engineeringand high performance polymers, the high levels of additives (greaterthan about 20% by weight, in most examples) that were required usuallyaltered the physical characteristics of the polymeric material to thepoint that the formed conductive composite material no longer exhibitedthe desired characteristics of the starting, non-conductive material.Thus, the examination of junctions formed with such materials has in thepast generally required the addition of an intervening material, such asa pressure sensitive film within the junction, leading to the problemsdiscussed above.

It should be noted, however, that while the presently disclosed sensorscan be of great benefit when formed to include engineering and/or highperformance polymeric composite materials, this is not a requirement ofthe invention. In other aspects, the polymer utilized to form thecomposite material can be a more conventional polymer. No matter whatpolymer, copolymer, or combination of polymers is used to form thedisclosed composite conductive materials, the composite materials of thedisclosed sensors can exhibit pressure sensitive electrical conductivityand if desired, can also be formed so as to essentially maintain thephysical characteristics of a polymeric material identical to thecomposite but for the lack of the conductive filler.

In general, any polymeric material that can be combined with anelectrically conductive filler to form a pressure sensitive conductivepolymeric composite material that can then be formed into an essentiallyinflexible shape can be utilized in the contact sensors describedherein. For example, various polyolefins, polyurethanes, polyesterresins, epoxy resins, and the like can be utilized in the contactsensors described herein. In certain aspects, the composite material caninclude engineering and/or high performance polymeric materials. In oneparticular aspect, the composite material can include UHMWPE. UHMWPE isgenerally classified as an engineering polymer, and possesses a uniquecombination of physical and mechanical properties that allows it toperform extremely well in rigorous wear conditions. In fact, it has thehighest known impact strength of any thermoplastic presently made, andis highly resistant to abrasion, with a very low coefficient offriction. The physical characteristics of UHMWPE have made it attractivein a number of industrial and medical applications. For example, it iscommonly used in forming polymeric gears, sprockets, impact surfacesbearings, bushings and the like. In the medical industry, UHMWPE iscommonly utilized in forming replacement joints including portions ofartificial hips, knees, and shoulders. In addition, UHMWPE can be inparticulate form at ambient conditions and can be shaped throughcompression molding or RAM extrusion and can optionally be machined toform an essentially inflexible block (i.e., not easily misshapen ordistorted), with any desired surface shape.

Conductive fillers as are generally known in the art can be combinedwith the polymeric material of choice to form the composite material ofthe disclosed sensors. The conductive fillers can be, for example andwithout limitation, carbon black and other known carbons, gold, silver,aluminum, copper, chromium, nickel, platinum, tungsten, titanium, iron,zinc, lead, molybdenum, selenium, indium, bismuth, tin, magnesium,manganese, cobalt, titanium germanium, mercury, and the like.

According to one aspect, a pressure sensitive conductive compositematerial can be formed by combining a relatively small amount of aconductive filler with a polymeric material. For example, the compositecan comprise from between about 0.1% to about 20% by weight of theconductive filler, more preferably from between about 1% to about 15% byweight of the conductive filler, and most preferably from between about5% to about 12% by weight of the conductive filler. Of course, in otheraspects, such as those in which the physical characteristics of thecomposite material need not approach those of the non-conductivepolymeric material, the composite material can include a higher weightpercentage of the conductive filler material.

In general, the polymeric material and the conductive filler can becombined in any suitable fashion, which can generally be determined atleast in part according to the characteristics of the polymericmaterial. For example, and depending upon the polymers involved, thematerials can be combined by mixing at a temperature above the meltingtemperature of the polymer (conventional melt-mixing) and the fillermaterials can be added to the molten polymer, for example, in aconventional screw extruder, paddle blender, ribbon blender, or anyother conventional melt-mixing device. The materials can also becombined by mixing the materials in an appropriate solvent for thepolymer (conventional solution-mixing or solvent-mixing) such that thepolymer is in the aqueous state and the fillers can be added to thesolution. Optionally, an appropriate surfactant can be added to themixture of materials to permit or encourage evaporation of the solvent,resulting in the solid conductive composite material. In another aspect,the materials can be mixed below the melting point of the polymer and indry form. In this aspect, the materials can be mixed by a standardvortex mixer, a paddle blender, a ribbon blender, or the like, such thatthe dry materials are mixed together before further processing.

When mixing the components of the composite material, the mixing can becarried out at any suitable conditions. For example, in one aspect, thecomponents of the composite material can be mixed at ambient conditions.In other aspects, however, the components of the composite material canbe mixed at non-ambient conditions. For example, the components of thecomposite material can be mixed under non-ambient conditions to, forexample and without limitation, maintain the materials to be mixed inthe desired physical state and/or to improve the mixing process.

When dry mixing the materials to be utilized in the composite, the exactparticulate dimensions of the materials are not generally critical tothe invention. However, in certain aspects, the relative particulatesize of the materials to be combined in the mixture can be important. Inparticular, the relative particulate size of the materials to becombined can be important in those aspects wherein a relatively lowamount of conductive filler is desired and in those aspects wherein thepolymer granules do not completely fluidize during processing. Forexample, the relative particle size can be important in certain aspectswherein engineering or high-performance polymers are utilized. It iscontemplated that the relative particle size can be particularlyimportant during utilization of extremely high melt viscosity polymerssuch as UHMWPE, which can be converted via non-fluidizing conversionprocesses, including, for example and without limitation, compressionmolding or RAM extrusion processes.

In such aspects, the particle size of the filler can beneficially beconsiderably smaller than the particle size of the polymer. According tothis aspect, and while not wishing to be bound by any particular theory,it is believed that due to the small size of the conductive fillerparticles relative to the larger polymer particles, the conductivefiller is able to completely coat the polymer during mixing and, uponconversion of the composite polymeric powder in a non-fluidizingconversion process to the final solid form, the inter-particle distanceof the conductive filler particles can remain above the percolationthreshold such that the composite material can exhibit the desiredelectrical conductivity. According to this aspect, when forming thecomposite mixture, the granule or aggregate size of the conductivefiller to be mixed with the polymer can be at least about two orders ofmagnitude smaller than the granule size of the polymer. In some aspects,the granule or aggregate size of the conductive filler can be at leastabout three orders of magnitude smaller than the granule size of thepolymer.

In forming the composite material according to this aspect, a granularpolymer can be dry mixed with a conductive filler that is also inparticulate form. Readily available UHMWPE in general can have a granulediameter in a range of from about 50 μm to about 200 μm. Typically, theindividual granule is made up of multiple sub-micron sized spheroids andnano-sized fibrils surrounded by varying amounts of free space.

In one aspect, the conductive filler for mixing with the polymer cancomprise carbon black. Carbon black is readily available in a widevariety of agglomerate sizes, generally ranging in diameter from about 1μm to about 100 μm that can be broken down into smaller aggregates offrom about 10 nm to about 500 nm upon application of suitable energy.

Upon dry mixing the particulate conductive filler with the largerparticulate polymer material with suitable energy, the smaller granulesof conductive filler material can completely coat the larger polymergranules. For example, a single powder particle can be obtainedfollowing mixing of 8 wt % carbon black with 92 wt % UHMWPE. Therefore,the UHMWPE particles are completely coated with carbon black aggregates.While not wishing to be bound by any particular theory, it is believedthat forces of mixing combined with electrostatic attractive forcesbetween the non-conductive polymeric particles and the smallerconductive particles are primarily responsible for breaking theagglomerates of the conductive material down into smaller aggregates andforming and holding the coating layer of the conductive material on thepolymer particles during formation of the composite powder as well asduring later conversion of the powdered composite material into a solidform.

Following formation of the mixture comprising the conductive filler andthe polymeric material, the mixture can be converted as desired to forma solid composite material. In one aspect, the solid composite materialcan be electrically conductive. The solid composite thus formed can alsomaintain the physical characteristics of the polymeric material inmixtures comprising a relatively low weight percentage of conductivefiller. For example, in the aspect described above, in which thecomposite material includes a conductive filler mixed with UHMWPE, thepowder can be converted via a compression molding process or a RAMextrusion process, as is generally known in the art. Optionally,following conversion of the powder, the resultant solid molded materialcan be machined to produce a desired curvature on at least one contactsurface.

In other aspects however, and primarily depending upon the nature of thepolymeric portion of the composite, other conversion methods maypreferably be employed. For example, in other aspects, the polymericportion of the composite material can be a polymer, a co-polymer, or amixture of polymers that can be suitable for other converting processes.For example and without limitation, the composite polymeric material canbe converted via a conventional extrusion or injection molding process.

The composite material of the disclosed sensors can optionally compriseother materials in addition to the primary polymeric component and theconductive filler discussed above. In one aspect, the composite materialcan comprise additional fillers, including, for example and withoutlimitation, various ceramic fillers, aluminum oxide, zirconia, calcium,silicon, fibrous fillers, including carbon fibers and/or glass fibers,or any other fillers as are generally known in the art. In anotheraspect, the composite material can include an organic filler, includingfor example and without limitation, tetrafluoroethylene or afluororesin. In this aspect, it is contemplated that the organic fillercan be added to improve sliding properties of the composite material.

It is believed that during the conversion process, the polymer particlescan fuse together and confine the conductive filler particles to athree-dimensional channel network within the composite, forming asegregated network type of composite material. In operation, thedistances between many of the individual carbon black primary particlesand small aggregates is quite small, believed to be nearing 10 nm. It iscontemplated that when two conductive filler particles are within about10 nm of each other, they can conduct current via electron tunneling, orpercolation, with very little resistance. Thus, many conductive pathsfulfilling these conditions can be traced across the image. Moreover, asthe polymers are deformable, the conductivity, and in particular theresistance, of the composite material of the contact sensors describedherein can vary upon application of a compressive force (i.e., load) tothe composite material.

Accordingly, following any desired molding, shaping, cutting and/ormachining and also following any desired physical combination of theformed composite material with other non-conductive materials (variousaspects of which are discussed further below), the composite materialsof the contact sensors described herein, which comprise at least oneconductive filler, can be formed into the sensor shape and placed inelectrical communication with a data acquisition terminal. For example,in one aspect, the composite material of the sensor can be connected toa data acquisition terminal In this aspect, the composite material canbe connected to the data acquisition terminal by, for example andwithout limitation, conventional alligator clips, conductive epoxy,conductive silver ink, conventional rivet mechanisms, conventionalcrimping mechanisms, and other conventional mechanisms for maintainingelectrical connections. In another aspect, the composite material can bemachined to accept a connector of a predetermined geometry within thecomposite material itself. Other connection regimes as are generallyknown in the art may optionally be utilized, however, including fixed orunfixed connections to any suitable communication system between thecomposite material and the data acquisition terminal In particular, noparticular electrical communication system is required of the contactsensors described herein. For example, in other aspects, the electricalcommunication between the composite material and the data acquisitionterminal can be wireless, rather than a hard wired connection.

In one aspect, the data acquisition terminal can comprise dataacquisition circuitry. In another aspect, the data acquisition terminalcan comprise at least one multiplexer placed in electrical communicationwith a microcontroller via the data acquisition circuitry. In anadditional aspect, the data acquisition circuitry can comprise at leastone op-amp for providing a predetermined offset and gain through thecircuitry. In this aspect, the at least one op-amp can comprise aconverting op-amp configured to convert a current reading into a voltageoutput. It is contemplated that the converting op-amp can measurecurrent after it has passed through the at least one multiplexer andthen convert the measured current into a voltage output. In a furtheraspect, the data acquisition terminal can comprise an Analog/Digital(A/D) converter. In this aspect, the A/D converter can be configured toreceive the voltage output from the converting op-amp. It iscontemplated that the A/D converter can convert the voltage output intoa digital output signal. In yet another aspect, the data acquisitionterminal can be in electrical communication with a computer having aprocessor. In this aspect, the computer can be configured to receive thedigital output signal from the A/D converter. It is contemplated thatthe A/D converter can have a conventional Wi-Fi transmitter forwirelessly transmitting the digital output signal to the computer. It isfurther contemplated that the computer can have a conventional Wi-Fireceiver to receive the digital output signal from the A/D converter.

As electrical communications methods and electrical data analysismethods and systems are generally known in the art, these particularaspects of the disclosed contact sensor systems are not described ingreat detail herein. FIG. 5 is a block diagram illustrating an exemplaryoperating environment for performing the disclosed methods and portionsthereof. This exemplary operating environment is only an example of anoperating environment and is not intended to suggest any limitation asto the scope of use or functionality of operating environmentarchitecture. Neither should the operating environment be interpreted ashaving any dependency or requirement relating to any one or combinationof components illustrated in the exemplary operating environment.

The present methods and systems can be operational with numerous othergeneral purpose or special purpose computing system environments orconfigurations. Examples of well known computing systems, environments,and/or configurations that can be suitable for use with the system andmethod comprise, but are not limited to, personal computers, servercomputers, laptop devices, hand-held electronic devices,vehicle-embedded electronic devices, and multiprocessor systems.Additional examples comprise set top boxes, programmable consumerelectronics, network PCs, minicomputers, mainframe computers,distributed computing environments that comprise any of the abovesystems or devices, and the like.

The processing of the disclosed methods and systems can be performed bysoftware components. The disclosed system and method can be described inthe general context of computer-executable instructions, such as programmodules, being executed by one or more computers or other devices.Generally, program modules comprise computer code, routines, programs,objects, components, data structures, etc. that perform particular tasksor implement particular abstract data types. In one aspect, the programmodules can comprise a system control module. The disclosed method canalso be practiced in grid-based and distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a communications network. In a distributed computingenvironment, program modules can be located in both local and remotecomputer storage media including memory storage devices.

Further, one skilled in the art will appreciate that the system andmethod disclosed herein can be implemented via a general-purposecomputing device in the form of a computer 200. As schematicallyillustrated in FIG. 5, the components of the computer 200 can comprise,but are not limited to, one or more processors or processing units 203,a system memory 212, and a system bus 213 that couples various systemcomponents including the processor 203 to the system memory 212.

The system bus 213 represents one or more of several possible types ofbus structures, including a memory bus or memory controller, aperipheral bus, an accelerated graphics port, and a processor or localbus using any of a variety of bus architectures. By way of example, sucharchitectures can comprise an Industry Standard Architecture (ISA) bus,a Micro Channel Architecture (MCA) bus, an Enhanced ISA (EISA) bus, aVideo Electronics Standards Association (VESA) local bus, an AcceleratedGraphics Port (AGP) bus, and a Peripheral Component Interconnects (PCI)bus also known as a Mezzanine bus. The bus 213, and all buses specifiedin this description can also be implemented over a wired or wirelessnetwork connection and each of the subsystems, including the processor203, a mass storage device 204, an operating system 205, contact sensorsoftware 206, contact sensor data 207, a network adapter 208, systemmemory 212, an Input/Output Interface 210, a display adapter 209, adisplay device 211, and a human machine interface 202, can be containedwithin one or more remote computing devices 214 a,b,c at physicallyseparate locations, connected through buses of this form, in effectimplementing a fully distributed system.

The computer 200 typically comprises a variety of computer readablemedia. Exemplary readable media can be any available media that isaccessible by the computer 200 and comprises, for example and not meantto be limiting, both volatile and non-volatile media, removable andnon-removable media. The system memory 212 can comprise computerreadable media in the form of volatile memory, such as random accessmemory (RAM), and/or non-volatile memory, such as read only memory(ROM). The system memory 212 typically contains data such as pressureand/or hysteresis data 207 and/or program modules such as operatingsystem 205 and contact sensor module software 206 that are immediatelyaccessible to and/or are presently operated on by the processing unit203.

In another aspect, the computer 200 can also comprise otherremovable/non-removable, volatile/non-volatile computer storage media.By way of example, FIG. 5 illustrates a mass storage device 204 whichcan provide non-volatile storage of computer code, computer readableinstructions, data structures, program modules, and other data for thecomputer 200. For example and not meant to be limiting, a mass storagedevice 204 can be a hard disk, a removable magnetic disk, a removableoptical disk, magnetic cassettes or other magnetic storage devices,flash memory cards, CD-ROM, digital versatile disks (DVD) or otheroptical storage, random access memories (RAM), read only memories (ROM),electrically erasable programmable read-only memory (EEPROM), and thelike.

Optionally, any number of program modules can be stored on the massstorage device 204, including by way of example, an operating system 205and contact sensor module software 206. Each of the operating system 205and contact sensor module software 206 (or some combination thereof) cancomprise elements of the programming and the load cell module software206. Pressure and/or hysteresis data 207 can also be stored on the massstorage device 204. Pressure and/or hysteresis data 207 can be stored inany of one or more databases known in the art. Examples of suchdatabases comprise, DB2®, Microsoft® Access, Microsoft® SQL Server,Oracle®, mySQL, PostgreSQL, and the like. The databases can becentralized or distributed across multiple systems.

In another aspect, the user can enter commands and information into thecomputer 200 via an input device (not shown). Examples of such inputdevices comprise, but are not limited to, a keyboard, pointing device(e.g., a “mouse”), a microphone, a joystick, a scanner, tactile inputdevices such as gloves, and other body coverings, and the like. Theseand other input devices can be connected to the processing unit 203 viaa human machine interface 202 that is coupled to the system bus 213, butcan be connected by other interface and bus structures, such as aparallel port, game port, an IEEE 1394 Port (also known as a Firewireport), a serial port, or a universal serial bus (USB).

In yet another aspect, a display device 211 can also be connected to thesystem bus 213 via an interface, such as a display adapter 209. It iscontemplated that the computer 200 can have more than one displayadapter 209 and the computer 200 can have more than one display device211. For example, a display device can be a monitor, an LCD (LiquidCrystal Display), or a projector. In addition to the display device 211,other output peripheral devices can comprise components such as aprinter (not shown) which can be connected to the computer 200 viaInput/Output Interface 210.

The computer 200 can operate in a networked environment using logicalconnections to one or more remote computing devices 214 a,b,c. By way ofexample, a remote computing device can be a personal computer, portablecomputer, a server, a router, a network computer, a peer device or othercommon network node, and so on. Logical connections between the computer200 and a remote computing device 214 a,b,c can be made via a local areanetwork (LAN) and a general wide area network (WAN). Such networkconnections can be through a network adapter 208. A network adapter 208can be implemented in both wired and wireless environments. Suchnetworking environments are conventional and commonplace in offices,enterprise-wide computer networks, intranets, and the Internet 215.

For purposes of illustration, application programs and other executableprogram components such as the operating system 205 are illustratedherein as discrete blocks, although it is recognized that such programsand components reside at various times in different storage componentsof the computing device 200, and are executed by the data processor(s)of the computer. An implementation of contact sensor software 206 can bestored on or transmitted across some form of computer readable media.Computer readable media can be any available media that can be accessedby a computer. By way of example and not meant to be limiting, computerreadable media can comprise “computer storage media” and “communicationsmedia.” “Computer storage media” comprise volatile and non-volatile,removable and non-removable media implemented in any method ortechnology for storage of information such as computer readableinstructions, data structures, program modules, or other data. Exemplarycomputer storage media comprises, but is not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by a computer.

In various aspects, it is contemplated that the methods and systemsdescribed herein can employ Artificial Intelligence techniques such asmachine learning and iterative learning. Examples of such techniquesinclude, but are not limited to, expert systems, case based reasoning,Bayesian networks, behavior based AI, neural networks, fuzzy systems,evolutionary computation (e.g. genetic algorithms), swarm intelligence(e.g. ant algorithms), and hybrid intelligent systems (e.g. expertinference rules generated through a neural network or production rulesfrom statistical learning).

It is contemplated that the contact sensors described herein canoptionally comprise one or more sensing points. In one aspect, thecontact sensor can include only a single sensing point. For example, theentire contact surface of the disclosed sensors can be formed of theconductive composite material. According to this aspect, the contactsensors can be utilized to obtain impact data and/or the total load onthe surface at any time. Such an aspect can be preferred, for example,in order to obtain total load or impact data for a member without thenecessity of having external load cells or strain gauges incommunication with the load-bearing member. This sensor type may beparticularly beneficial in those aspects wherein the sensor is intendedto be incorporated with or as the member for use in the field. Forexample, any polymeric load-bearing member utilized in a process couldbe formed from the physically equivalent but conductive compositematerial as described herein and incorporated into the working processto provide real time wear and load data of the member with no cost inwear performance to the member due to the acquisition of conductivecapability.

In other aspects, the sensors disclosed herein can include a pluralityof sensing points and can provide more detailed data about the junctionor the members forming the junction. For example, the plurality ofsensing points can provide data describing the distribution of contactstresses and/or internal stresses, data concerning types of wear modes,or data concerning a lubrication regime as well as load and impact datafor a member forming a junction. According to this aspect, the compositematerial can be located at predetermined, discrete regions of a sensorto form the plurality of sensing points on or in the sensor, and anon-conductive material can separate the discrete sensing points fromone another. Data from the plurality of discrete sensing points can thenbe correlated and analyzed and can provide information concerning, forexample, the distribution of contact characteristics across the entiremating surface, and in particular can provide contact information underdynamic loading conditions involving, for example, sliding, rolling, orgrinding motions across the surface of the sensor.

It is contemplated that the plurality of sensing points can be arrangedin any desired configuration along a surface of the sensor. For example,and without limitation, the sensing points can be positioned in a seriesof parallel rows. Alternatively, the sensing points can be positioned instaggered or overlapping configurations. In one aspect, the sensingpoints can be substantially evenly spaced. In another aspect, thesensing points can be substantially unevenly spaced.

It is contemplated that selected sensing points among the plurality ofsensing points can be activated during the application of a load whilethe remainder of the sensing points remain deactivated.

FIG. 1 is a schematic diagram of one aspect of the sensor as disclosedherein, including a plurality of sensing points at the essentiallyinflexible contact surface of the junction member. Surface sensingpoints such as those in this aspect can be utilized to determine contactsurface data, including, for example, contact stress data, lubricationdata, impact data, and information concerning wear modes. The polymericsensor 10 includes a contact surface 8 for contact with a metalliccomponent (not shown) to simulate the dynamic characteristics of thejoint formed between the sensor and the metallic component. In thisparticular aspect, the contact surface 8 describes a curvature tosimulate that of the tibial plateau of an artificial knee implant.

As can be seen with reference to FIG. 1, the sensor 10 includes aplurality of sensing points 12 at the contact surface 8 of the sensor10. The sensing points 12 can be formed of the conductive compositematerial as herein described. Thus, unlike conventional contact sensors,the conductive composite material functions as not only the sensingmaterial, but also as an electrical communication pathway. Each sensingpoint is configured to produce an output signal in response to thechange in resistance experienced by the conductive composite material atthe sensing point.

In one aspect, because the conductive composite material provideselectrical communication between the sensing points at the contactsurface of the sensor, the conductive composite material of each sensorcan have a bulk resistance. In this aspect, the bulk resistance can bemeasured in Ohms per unit length; accordingly, as the length of thesensor increases, the bulk resistance proportionally increases.Therefore, the bulk resistance of the conductive composite materialvaries from one sensing point to another sensing point. It iscontemplated that the farther a particular sensing point is from anelectrical connection between the sensor and the data acquisitionterminal, the greater the bulk resistance will be at that particularsensing point. Consequently, it is contemplated that the resistancemeasured at each sensing point will be different even when the change inresistance at some sensing points is identical. In addition, it iscontemplated that the sensing points can always have at least some levelof electrical communication with adjacent sensing points, even when aload is not being applied. Thus, when a load is applied to one or moresensing points, the sensing points that are subjected to the load cangenerate current within sensing points that are not subjected to theload (parallel resistance paths).

In order to account for the bulk resistance of the composite materialand the parallel resistance paths described herein, the processor of thecomputer disclosed herein can be programmed to accurately determine theactual change in contact resistance experienced at each sensing point ofthe sensor based on the digital output signal received from the A/Dconverter of the data acquisition terminal. In one aspect, the processorcan be configured to calculate contact resistance changes at individualsensing points based on the current measurements at each respectivesensing point. In this aspect, the processor can calculate theresistance changes as the solution to a series of non-linear equationsthat describe the load in terms of the current measurements at eachrespective sensing point. It is contemplated that the processor can beconfigured to solve the series of simultaneous non-linear equationsusing one or more conventional algorithms, including, for example andwithout limitation, the “Newton-Raphson method” and the “node analysis”method. The contact resistance changes calculated by the processor canthen be used to determine the actual applied load at each respectivesensing point.

It is contemplated that the conductive paths produced by the pluralityof sensing points can vary depending on the spatial arrangement of thesensing points. For example, the conductive paths produced by sensingpoints in a parallel and evenly spaced configuration will besubstantially different than the conductive paths produced when thesensing points are positioned in overlapping, staggered, or unevenlyspaced configurations.

In use, and with reference to FIG. 1, upon contact of a single sensingpoint 12 with the metallic component, an electrical signal can begenerated and sent via wire 18 to a data acquisition terminal asdescribed herein. In one aspect, this electrical signal can be sent inresponse to a voltage excitation signal that is processed to theelectrical signal by the data acquisition terminal. As one skilled inthe art will appreciate, in this example, the metallic component isacting as a first electrode that is mechanically and electricallycoupling to the polymeric composite material, which is in turnelectrically coupled to a second electrode, i.e., the wire 18. Theelectrically coupled respective first and second electrodes and thepolymeric composite material forms an electrical circuit. Though notexpressly shown in the Figure, in this particular aspect, each sensingpoint 12 of the plurality of sensing points can be wired so as toprovide data from that point to the data acquisition terminal Due to thedeformable nature of the polymeric composite material, thecharacteristics of the generated electrical signal can vary with thevariation in load applied at the sensing point 12 and a dynamic contactstress distribution profile for the joint can be developed.

The surface area and geometry of any individual sensing point 12 as wellas the overall geometric arrangement of the plurality of sensing points12 over the surface 8 of the sensor 10, can be predetermined as desired.For example, through the formation and distribution of smaller sensingpoints 12 with less intervening space between individual sensing points12, the spatial resolution of the data can be improved. While there maybe a theoretical physical limit to the minimum size of a single sensingpoint determined by the size of a single polymer granule, practicallyspeaking, the minimum size of the individual sensing points will only belimited by modern machining and electrical connection formingtechniques. In addition, increased numbers of data points can complicatethe correlation and analysis of the data. As such, the preferredgeometry and size of the multiple sensing points can generally involve acompromise between the spatial resolution obtained and complication offormation methods.

In this particular aspect as seen in FIG. 1, the composite materialforming the surface sensing points 12 can extend to the base 15 of thesensor 10, where electrical communication can be established to a dataacquisition and analysis module, such as a computer with suitablesoftware, for example.

In one aspect, the discrete sensing points 12 of the sensor 10 of FIG. 1can be separated by a non-conductive material 14 that can, in oneaspect, be formed of the same polymeric material as that contained inthe composite material forming the sensing points 12. In general, themethod of combining the two materials to form the sensor can be anysuitable formation method. For example, in one aspect the compositematerial can be combined with a virgin material to produce one or moresensor sheets as described herein. Alternatively, the composite materialcan be formed into a desired shape, such as multiple individual rods ofcomposite material as shown in the aspect illustrated in FIG. 1, andthen these discrete sections can be inserted into a block of thenon-conductive polymer that has had properly sized holes cut out of theblock. Optionally, the two polymeric components of the sensor can thenbe fused, such as with heat and/or pressure, and any final shaping ofthe two-component sensor, such as surface shaping via machining, forexample, can be carried out so as to form the sensor 10 includingdiscrete sensing points 12 formed of the conductive composite materialat the surface 8.

In many aspects of the invention, the same material can be used but forthe presence or absence of the conductive filler for the compositesensing points 12 as for the intervening spaces 14 since, as describedabove, the physical characteristics of the composite material can beessentially identical to the physical characteristics of thenon-conductive material used in forming the composite. According to thisaspect, the sensor 10 can have uniform physical characteristics acrossthe entire sensor 10, i.e., both at the sensing points 12 and in theintervening space 14 between the composite materials.

In one particular aspect, the polymer used to form the sensor 10 can bethe same polymer as is used to form the member for use in the field. Forexample, when considering the examination of artificial joints, thepolymer used to form both the composite material at the sensing points12 and the material in the intervening space 14 between the sensingpoints 12 can be formed of the same polymer as that expected to be usedto form the polymeric bearing component of the implantable device (e.g.,UHMWPE or polyurethane). Thus, the sensor 10 can provide real time,accurate, dynamic contact data for the implantable polymeric bearingunder expected conditions of use.

Optionally, the surface 8 of the sensor 10 can be coated with alubricating fluid, and in particular, a lubricating fluid such as may beutilized for the bearing during actual use and under the expectedconditions of use (e.g., pressure, temperature, etc.). In this aspect,in addition to providing direct contact data, the disclosed sensors canalso be utilized to examine data concerning contact through anintervening material, i.e., lubrication regimes under expectedconditions of use. For example, the sensor can be utilized to determinethe type and/or quality of lubrication occurring over the surface of thesensor including variation in fluid film thickness across the surfaceduring use. In one aspect, this can merely be determined by presence orabsence of fluid, e.g., presence or absence of direct contact data(i.e., current flow) in those aspects wherein the fluid is anon-conductive lubricating fluid. In other aspects, a more detailedanalysis can be obtained, such as determination of variation in fluidfilm thickness. This information can be obtained, for example, bycomparing non-lubricated contact data with the data obtained from thesame joint under the same loading conditions but including theintervening lubricant. In another aspect, such information could beobtained through analysis of the signal obtained upon variation of thefrequency and amplitude of the applied voltage. In yet another aspect,the sensor can be utilized in a capacitance mode, in order to obtain theexact distance between the two surfaces forming the joint. In oneparticular aspect, the disclosed sensor can be utilized to determine alubrication distribution profile of the contact surface over time.

FIG. 2 illustrates another aspect of the contact sensors as describedherein. According to this aspect, the sensor includes multiple sensingstrips 16 across the surface 8 of the sensor 10. As illustrated, in thisaspect, the orientation of the individual sensing strips 16 across thedifferent condoyles formed on the single sensor surface can be variedfrom one another. Alternatively, and as shown in FIG. 3, strips can belaid in different orientations on separate but identically shapedsensors in a multi-sensor testing apparatus. In any case, by varying theorientation of sensor strips on multiple, but essentially identicalsurfaces, virtual cross-points can be created when the data from thedifferent surfaces is correlated. In particular, when contacts of thesame shape and magnitude at the same location of different surfaces arerecognized, a virtual data point at the cross-point can be created. Ascan be seen in FIG. 3, this aspect can allow the formation of fewerelectrical connections and wires 18 in order to provide data to theacquisition and analysis location, which may be preferred in someaspects due to increased system simplicity.

Optionally, it is contemplated that the contact sensors as describedherein can be utilized to provide sub-surface stress data. For example,in the aspect illustrated in FIG. 3, multiple sensing strips 16 can belocated within a subsurface layer at a predetermined depth of thesensor. According to this aspect, the horizontal and vertical strips 16can cross each other with a conductive material located between thecross points to form a subsurface sensing point 15 at each cross point.In one aspect, the strips 16 can be formed of the composite materialdescribed herein with the intervening material being the same basiccomposite material but with a lower weight percentage of the conductivefiller, and the layer can be laid within the insulating non-conductivepolymer material 14. In another aspect, the sensing strips 16 can be anyconductive material, such as a metallic wire, for example, laid oneither side of a sheet or section of the composite material and thelayer can then be located at a depth from the surface 8 of the sensor.

Application of a load at the surface 8 of the sensor can then vary theelectronic characteristics at the internal sensing point 15. Inparticular, the current flow at any point 15 can vary in proportion tothe stress at that point. Thus, when data from multiple sensing points15 are correlated, an internal stress profile for the sensor can bedeveloped at that particular depth.

In yet another aspect, in lieu of strips, the conductive filler may bearranged on the sensor sheet as a plurality of dots, as shown in FIG.13. In this aspect, there would be reduced opportunity for cross-talkwhen the sensor sheets were thermoformed into shape. In this aspect, itcan be appreciated that the electrical connections necessary to performthe load analysis can be challenging due to the number of connectionsrequired. As such, application of current to one of the sheets may beachieved using a sheet of flexible conductive material, such as, forexample, mesh or foil. In use, a sensor sheet having a plurality ofconductive dots can be configured for coupling with electrodes proximateeach respective conductive dots. Following application of a load with ametallic or other conductive element, it is contemplated that currentcan flow through the conductive filler therein the sensor sheet, therebypermitting calculation of the applied loads.

FIG. 14 is a schematic of a contact sensor in operative communicationwith a data acquisition terminal, and showing a battery operativelycoupled to the data acquisition terminal and a computer coupled to thedata acquisition terminal via a Wi-Fi transmitter.

In use, it is contemplated that a plurality of sensor sheets can bethermoformed in substantially identical three-dimensional sizes andorientations. In one aspect, the sensor sheets can be placed in astacked relationship with adjacent sensor sheets. In this aspect, it iscontemplated that no fusing between adjacent sensor sheets will occur.In another aspect, the configurations of the portions of conductivefiller therein the sensor sheets can be selected to create overlapbetween the conductive portions of adjacent sensor sheets. For example,and without limitation, the conductive portions of one sensor sheet canbe oriented substantially perpendicularly to the conductive portions ofan adjacent sensor sheet prior to stacking of the sensor sheets. It iscontemplated that upon application of a load to the sensor sheets, eachrespective sensor sheet can function as an electrode such that noadditional contact with a conductive element is required to producecurrent therethrough the sensors sheets. It is further contemplated theoverlap between the conductive portions of the sensor sheets can createcross points for measuring loads applied to the sensor sheets.

In another aspect, as shown in FIG. 4, the sensors can include multiplestacked polymer sensor sheets. In this aspect, each polymer sensor sheethas a plurality of conductive stripes of conductive material that areseparated by non-conductive polymeric stripes. In the illustratedexample, the vertical conductive stripes on one sheet, “columns,” andthe horizontal conductive stripes on the underlying sheet, “rows,” arepositioned relative to each other so that, at the places where thesecolumns and rows spatially intersect, the conductive areas of the twosheets are in physical and electrical contact with each other. In oneaspect, the exemplary interface electronics illustrated in FIG. 15 canbe used with appropriate control software within the data acquisitionterminal to connect one column to a voltage source and one row to acurrent-to-voltage circuit, in order to measure the current through theconductive polymer materials. In one aspect, it is contemplated thateach column/row pair, i.e., the internal junction points 15, can bemeasured, one at a time, to provide a complete set of currentmeasurements. As illustrated, the stacked sensor sheets being orientedsubstantially perpendicular to each other allow for the formation of anarray of sensing points by the overlapping portions of the conductivestripes of the stacked sensor sheets.

In one aspect, these current measurements do not represent the currentsat the pressure-sensitive points in the stacked polymer sheets where thestripes overlap. Rather, the current measurements are actually externalmeasurements at external points (also called “nodes”), which aregenerally near the outer edges of the material. The measurement data areprocessed in software within the data acquisition terminal in order tocalculate the individual currents that are present at each measurementpoint where the columns and rows overlap, and then this information isused to determine the pressure that is applied at each measurementpoint. An exemplary, non-limiting, schematic of the measurementcircuitry is provided in FIG. 16 herein.

In yet another aspect, both subsurface contact data and surface contactdata can be gathered from a single sensor through combination of theabove-described aspects.

In one aspect, the sensor may comprise a thermoformable polymer, suchas, for example and without limitation, ultra high molecular weightpolyethylene (UHMWPE), high density polyethylene (HDPE), polyphenolynesulfide (PPS), low density polyethylene (LDPE), or polyoxymethylenecopolymer (POM). In this aspect, the sensor can be formed into the shapeof at least a portion of an artificial joint bearing. For example andnot meant to be limiting, a portion of a prosthetic limb. Pressuremapping of portions of a joint bearing can provide data necessary to fitthe prosthetic limb to the user with lower wear. In this aspect, apolymer capable of stretching is advantageous due to the non-uniformityof the shape of the prosthesis.

In another aspect, the sensors can be manufactured in a two stageprocess. First, the non-conductive sheets of thermoformable polymer canbe molded from raw material. Second, the conductive strips can be addedto the sensor sheet and placed back into the same mold. In this manner,flow of the non-conductive polymer into the conductive region of thesheet, and flow of the polymer with conductive filler into thenon-conductive region of the sheet can be minimized to ensure that, whenthermoformed, there is no cross-talk between adjacent conductive strips.

In this aspect, calibration of the each sensor can be performed prior tothe thermoforming step, as calibration after thermoforming can prove tobe more difficult. It is believed that the characteristics of thesensors do not substantially change during the thermoforming process.

In one aspect, such calibration may be desired as each individual sensorcan have individually unique electrical properties that must becalibrated to a standard in order to achieve a desired degree of loadmeasurement accuracy. Further, it is believed that the individualsensors can experience hysteresis when the sensors are unloaded. Thus,it is contemplated that conventional signal processing componentsconfigured to correlate the voltage or current to the load of therespective sensor can be implemented using software configured tocorrelate the load during loading and load during unloading. Tocompensate for the observed hysteresis effect, it is also contemplatedthat the software can be configured to calculate the load during astatic position—when the load is substantially constant—by using a meanpoint between a calculated load value during loading and a calculatedload value during unloading.

When used in making a prosthetic limb, for example, the sensor sheetscan be thermoformed into the shape of a cup for receiving the anatomicallimb. Once the sheets are used to map out the force distribution in thecup, the sensor sheets can be adjusted accordingly. This process can berepeated until the forces are substantially uniformly distributed asdesired. Once the desired level of force distribution is achieved, amold, such as for example, a plaster mold, can be made of the interiorportion of the cup. Then the mold can be used to form the cup out ofmaterials that are suitable for the prosthesis.

Optionally, the composite materials produced as described herein can beincorporated into one or more sensor sheets. In one aspect, a method forproducing the sensor sheets can comprise providing a plurality ofsubstantially circular virgin sheets comprising at least one virginmaterial. In this aspect, the virgin material can comprise, for exampleand without limitation, virgin UHMWPE. In another aspect, the method forproducing the sensor sheets can comprise providing a plurality ofsubstantially circular composite sheets comprising at least onecomposite material as disclosed herein. In this aspect, the compositematerial can comprise, for example and without limitation, a mixture ofcarbon black and UHMWPE. In an additional aspect, the virgin sheets canhave an outer diameter substantially equal to an outer diameter of thecomposite sheets. In yet an another aspect, the virgin sheets can havean inner diameter substantially equal to an inner diameter of thecomposite sheets. In a further aspect, the method for producing thesensor sheets can comprise positioning the virgin sheets and thecomposite sheets can be stacked in a desired configuration. In thisaspect, the desired configuration can comprise a single stack ofalternating virgin and composite sheets such that virgin sheets areintermediate and in contact with composite sheets and composite sheetsare intermediate and in contact with virgin sheets.

In an additional aspect, while the virgin and composite sheets arestacked in the desired configuration, the virgin and composite sheetscan be subjected to a conventional compression molding process forheating and then fusing the virgin and composite sheets together. Inanother aspect, the compression molding of the virgin and compositesheets can produce a substantially cylindrical billet. In this aspect,the substantially cylindrical billet can be substantially hollow. In afurther aspect, the billet can be placed on a conventional mandrel. Inthis aspect, the mandrel can be configured to spin at a desired rate. Instill a further aspect, the method for producing the sensor sheets cancomprise spinning the mandrel, thereby turning the billet as the mandrelspins. In another aspect, the method can comprise subjecting the billetto a conventional skiving machine. It is contemplated that the skivingmachine can comprise a blade for slicing or shaving off a thin layer ofthe billet. In operation, the blade of the skiving machine advancestoward the billet at a constant rate as the billet rotates on themandrel, thereby producing the sensor sheets. In one aspect, the sensorsheets can be of substantially uniform thickness. In this aspect, it iscontemplated that the sensor sheets can have a thickness ranging fromabout 0.001 inches to about 0.050 inches, more preferably from about0.002 inches to about 0.030 inches, and most preferably from about 0.003inches to about 0.020 inches.

The contact sensors described herein may be better understood withreference to the Examples, below.

EXAMPLE 1

An industrial-grade UHMWPE powder (GUR 1150, available from TiconaEngineering Polymers) having a molecular weight of 6×10⁶, density of0.93 g/mL, T_(m) of 135° C., and an average particle size of 100 μm, wascombined with carbon black (CB) (Printex L-6 available from DegussaHulls, Dusseldorf, Germany) having a primary particle size of 18 nm anddibutyl phthalate absorption of 120 mL/100 g. Amounts of each powderwere placed in a 120 mL plastic sample container and initially manuallyshaken for 5 minutes to obtain four different samples having CB weightpercentages of 0.25%, 0.5%, 1%, and 8%. The samples were then mixed for10 minutes on a common laboratory vortex at the maximum speed setting.

Virgin UHMWPE powder and the four UHMWPE/CB powder mixtures were thencompression-molded into rectangular sheets 12 cm long, 8.5 cm wide, and2 mm thick using a mold consisting of a 2 mm thick Teflon framesandwiched between 2 stainless steel plates that were coated with Teflonmold release spray. The powders were processed in a laboratory press(Carver Laboratory Press, Model C, Fred S. Carver Inc., Wabash, Ind.)equipped with electric heaters for 20 minutes at a temperature of 205°C. and a pressure of 10 MPa. The specimens were then quenched underpressure at a cooling rate of 50° C. /min.

Tensile tests were performed to obtain stress-strain curves for eachcomposite and for the control. Results can be seen in FIG. 6 for thecontrol (20), 0.25% CB (22), 0.50% CB (24), 1.0% CB (26), and 8.0% CB(28). From these stress-strain curves, the modulus of elasticity wasdetermined for each composite, and these values were compared to thoseobtained for the control specimen. Both the control specimens and thecomposite specimens were formed from the same stock of virgin UHMWPEpowder (GUR 1150) by using the same processing parameters oftemperature, pressure, time, and cooling rate. The results from thetensile tests can be seen in Table 3, below. It was determined thatthere was no statistically significant difference (p=0.32, α=0.05)between the modulus of the 8% composite and the modulus of the virginUHMWPE control samples that were tested.

0.25 wt % 0.50 wt % 1 wt % 8 wt % n = 4 Control CB CB CB CB Young'sModulus 214.8 ± 21.1  208.48 ± 7.68  211.9 ± 7.74  212.6 ± 6.82  208.9 ±11.1  (MPa) Tensile Strength 30.8 ± 3.98 29.1 ± 2.23 32.6 ± 3.49 31.9 ±2.43 31.7 ± 1.03 (MPa) Yield Strength 17.8 ± 0.75 18.0 ± 0.87 17.8 ±0.93 15.2 ± 0.96 22.2 ± 1.07 (MPa) Elongation at  390 ± 77.0  360 ± 18.0 390 ± 18.0  340 ± 23.0  290 ± 41.0 Break (%)

The elastic modulus values obtained were comparable to those obtained byParasnis and colleagues for thin-film UHMWPE specimens (see Parasnis C,Ramani K. Analysis of the effect of pressure on compression molding ofUHMWPE. Journal of Materials Science: Materials in Medicine, Vol. 9, p165-172, 1998, which is incorporated herein by reference). The valuesobtained for tensile strength, yield strength, and elongation at breakcompared closely to the values cited in the literature (for example, seeis Li S. Burstein A. H., Current Concepts Review: Ultra-high molecularweight polyethylene. The Journal of Bone and Joint Surgery, Vol. 76-A,No. 7, p 1080-1090, 1994, which is incorporated herein by reference).

FIG. 7 shows a plot of the log of the resistance as a function of thelog of the compressive load applied to the UHMWPE/CB composites of 0.5%(24), 1% (26), and 8% (28). The plot shows that the composites have thesame slope, but that the intercepts are different, with the 0.5%composite having the highest intercept, and the 8% composite having thelowest intercept. The value of resistance changed by about two orders ofmagnitude for each composite. The correlation coefficients of eachregression line indicated a good fit. When the values of resistance werenormalized (shown in FIG. 8), the curves for the three composites werevery similar, suggesting that the amount of CB only affected themagnitude of the resistance. Thus, the relative response to applied loadappeared to be independent of the amount of CB. It should be noted thatthe control sample and the 0.25% CB sample had high resistance for allloads tested and thus were not included on FIGS. 7 and 8.

FIG. 9 shows the voltage values corresponding to the compressive load,the compressive displacement, and the resistance of the 8 wt % CBcomposite while the composite was loaded cyclically with a haversinewave at 1 Hz. The top curve corresponds to the compressive stress, themiddle curve corresponds to the compressive strain, and the bottom curvecorresponds to the resistance of the sensor material. This datarepresents the cyclic response of the material, indicating that it doesnot experience stress-relaxation at a loading frequency of 1 Hz. Theresults of this cyclic testing show that the peak voltage valuescorresponding to resistance remain nearly constant over many cycles.Therefore, the data seem to indicate that the sensor material should bewell suited for cyclic measurements since the readings do not degradeover time.

Monitoring the electrical resistance of the composite material whileapplying a compressive load revealed the force-dependent nature of theelectrical properties of the material. Because of the nano-scaledispersion of the conductive filler, the material's electrical responseto applied load was nearly ideal for all of the percentages tested. Thatis, the log of the material's resistance varied linearly with respect tothe log of the applied load. This linear relationship makes the materialwell suited for use as a sensor.

The data show that the linear relationship holds true for 0.5%, 1%, and8%, with the difference between the three being the value of theresistance. As all three percentages showed good sensor properties,specific formulations could be developed based on other criteria, suchas the specifics of the measurement electronics.

EXAMPLE 2

Compression molding was used to form 2 rectangular blocks of 1150 UHMWPEdoped with 8 wt % carbon black filler as described above for Example 1.The blocks formed included a 28×18 matrix of surface sensing points 12as shown in FIG. 1. The points were circular with a 1/16^(th) inch (1.59mm) diameter and spaced every 1/10^(th) inch (2.54 mm) The blocks werethen machined to form both a highly-conforming, PCL-sacrificing tibialinsert (Natural Knee II, Ultra-congruent size 3, CenterpulseOrthopedics, Austin, Tex.) and a less conforming PCL-retaining tibialinsert (Natural Knee II, Standard-congruent, size 3, CenterpulseOrthopedics, Austin, Tex.) as illustrated in FIG. 1. The implants werethen aligned and potted directly in PMMA in the tibial fixture of amulti-axis, force-controlled knee joint simulator (Stanmore/Instron,Model KC Knee Simulator). Static testing was performed with an axialload of 2.9 kN (4.times.B.W.) at flexion angles of 0°, 30°, 60°, and80°, to eliminate the effects of lubricant and to compare the sensorreading to the literature. The dynamic contact area was then measuredduring a standard walking cycle using the proposed 1999 ISOforce-control testing standard, #14243. Data was collected and averagedover 8 cycles. A pure hydrocarbon, light olive oil was used as thelubricant due to its inert electrical properties.

Static loading of the sensors showed that the contact area of the ultracongruent insert was significantly higher than that of the standardcongruent insert at all angles of flexion tested. The data closelyagreed with results found in the literature from FEA analysis.

The results from dynamic testing with a standard walking protocol, shownin FIG. 12, show the effects that the lubricant had on the dynamiccontact area. Contact area was registered by the sensor when physicalcontact occurred between the femoral component and any sensing point,allowing electrical current to flow. Because the lubricant waselectrically insulating, fluid-film lubrication over a sensing pointcaused no contact to be registered at that point. The lower contact areameasured for the ultra-congruent insert during the stance phase of gaitwas due to the fluid-film lubrication that occurred with the moreconforming insert. The rapid changes in contact area measured for thestandard-congruent insert during the mid-stance phase suggests that themode of lubrication is quite sensitive to the dynamic loading patterns.

EXAMPLE 3

Tecoflex SG-80A, a medical grade soft polyurethane available fromThermedics Inc. (Woburn, Mass.), was solution processed and moldedincluding 4 wt % and 48 wt % CB to form two solid sample materials.FIGS. 11A and 11B graphically illustrate the resistance vs. compressiveforce applied to the samples for the 4% and 48% non-surfactant mixedsamples, respectively. As can be seen, both samples showed pressuresensitive conductive characteristics suitable for forming the sensors asdescribed herein where the value of resistance can be controlled withthe amount of conductive filler added.

EXAMPLE 4

A 6″×6″ mold was constructed from normalized, pre-hardened 4140 steelwith a Rockwell hardness of HRC 32-35. The mold was designed and builtto mold 6×6 inch sensor sheets at approximately ⅛^(th) inch thick, andis shown in FIG. 12.

The mold was used to form “virgin” non-conductive sheets from raw highdensity polyethylene (HDPE) in powder form, similar to the fashion toform the sheets of UHMWPE in Example 1. HDPE works well in applicationsin which the sensor sheets need to be thermoformed. However, HDPE's lowgel viscosity makes it a challenge to keep adjacent regions of thesensor sheet separated from one another when forming the sensor sheet.

Although HDPE's melt temperature is readily available, observations weremade to confirm how the sensor sheets would behave in the mold. Athermocouple was used to measure the temperature in the oven. It wasdetermined that the transition temperature of the HDPE was 255°Fahrenheit. The mold was heated by upper and lower platens, which alsoapply compressive force. It was noted that, even with the correcttemperature being applied, some smearing could occur in the sensor sheetif compressive forces were not applied evenly. Any flow of the gelcaused smearing.

Once the virgin sheets were constructed, the conductive regions wereadded and the sheets were placed back into the same mold. Since neitherthe mold nor the press were perfectly square, the sheets that wereproduced varied by 10-20 thousands of an inch. To minimize thesevariances, the mold sections and the press sections were labeled on eachcorner. Once the mold was labeled, different mold and press alignmentswere tested to determine the alignments the produced the sheets with theleast variance.

Raw material in powder form was melted in the mold under the optimalalignment determined previously to produce the virgin sensor sheet. Theconductive filler portions were placed on the virgin sensor sheet. Then,the sheet with the conductive filler was placed back into the mold withthe same alignment to ensure that any dimensional variance that werepresent during the initial molding will also be present for the secondmolding. It was found that this procedure reduced material flow insidethe mold during gel state, which reduced smearing.

These and other modifications and variations to the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention, which ismore particularly set forth in the appended claims. In addition, itshould be understood that aspects of the various aspects may beinterchanged either in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention sofurther described in such appended claims.

We claim:
 1. A contact sensor, comprising: a data acquisition terminal;a plurality of polymer sensor sheets, each polymer sensor sheet havingselected spaced conductive portions, wherein each spaced conductiveportion is in communication with the data acquisition terminal, whereinadjacent polymer sensor sheets are stacked such that portions of theconductive portions therein the sensor sheets overlap and create anarray of sensing points at the overlapping portions of the stackedsensor sheets, and means for determining the pressure applied at leastone sensing point of the array of sensing points.
 2. The contact sensorof claim 1, wherein the conductive portions of each polymer sheet formconductive stripes extending the substantial length of the polymersheet.
 3. The contact sensor of any of the claims above, wherein theconductive stripes in one sensor sheet are oriented substantiallyperpendicular to the conductive portions of an adjacent sensor sheet. 4.The contact sensor of claim 3, wherein data acquisition terminal isprogrammed to selectively connect one conductive stripe of an underlyingpolymer sheet to a voltage source and one conductive stripe of theoverlying polymer sheet to a current-to-voltage circuit to measure thecurrent through the sensing point formed at the overlapping portions ofthe stacked sensor sheets.
 5. The contact sensor of any of the aboveclaims, wherein the data acquisition terminal is programmed to measurethe current at each sensing point of the array of sensing points.
 6. Thecontact sensor of any of the above claims, wherein the data acquisitionterminal is programmed to process the current measurements at at leastone sensing point to determine the pressure that is applied at eachsensing point.
 7. The contact sensor of any of the above claims, whereinthe polymer sensor sheet is formed from a substantially inflexiblecomposite material.
 8. The contact sensor of any of the above claims,wherein the conductive portion of each polymer sensor sheet is formedfrom a pressure sensitive conductive composite material that comprisesan electrically conducive filler and a polymeric material.
 9. Thecontact sensor of any of the above claims, wherein the non-conductivepotion of each polymer sheet is formed from a polymeric material. 10.The contact sensor of any of the above claims, wherein the polymericmaterial used in the conductive and non-conductive portions are the samepolymeric material.
 11. The contact sensor of any of the above claims,wherein the polymeric material is a thermoformable polymer.
 12. Thecontact sensor of any of the above claims, wherein the polymericmaterial is selected from a group consisting of: ultra high molecularweight polyethylene (UHMWPE), high density polyethylene (HDPE),polyphenolyne sulfide (PPS), low density polyethylene (LDPE), orpolyoxymethylene copolymer (POM).
 12. The contact sensor of any of theabove claims, wherein a desired amount of conductive filler can rangefrom about 0.1% to about 20% by weight of the pressure sensitivecomposite material.
 14. The contact sensor of any of the above claims,wherein a desired amount of conductive filler can range from about 1% toabout 15% by weight of the pressure sensitive composite material. 15.The contact sensor of any of the above claims, wherein a desired amountof conductive filler can range from about 5% to about 12% by weight ofthe pressure sensitive composite material.
 16. The contact sensor of anyof the above claims, wherein the conductive filler comprises carbonblack.
 17. The contact sensor of any of the above claims, wherein thepressure sensitive composite material further comprises ceramic fillers,aluminum oxide, zirconia, calcium, silicon, fibrous fillers, carbonfibers, glass fibers, and/or organic fillers.
 18. The contact sensor ofany of the above claims, wherein the contact sensor can be formed intothe shape of at least a portion of an artificial joint bearing.